The present invention relates to an improved asphalt-aggregate paving composition.
Aggregate-containing asphalt has been employed as the paving composition for roads or the like for many years. The asphalt includes bitumen as a predominant constituent and is conventionally obtained as a solid residue from the distillation of crude petroleum. The asphalt is converted to a fluid state when paving a road. One fluid form is the suspension or emulsion of the asphalt in water. After spreading and compressing the aggregate-containing asphalt, water evaporates and the asphalt hardens into a continuous mass. Another form of asphalt employed in road construction is a cutback, i.e., a liquid petroleum product produced by fluxing an asphaltic base with a suitable distillate. A road is formed by layering the cutback and evaporating the volatile distillate from the mass. The advantage of using the above road construction techniques is the avoidance of high temperature application. In an alternative technique, the asphalt and aggregate can be mixed and applied at elevated temperatures at the fluid state of the asphalt to form the road. This form of asphalt which is neither cut-back nor emulsified is referred to as asphalt cement.
A major problem with cutbacks and emulsions is their low adhesivity for aggregate in comparison to asphalt cement. This is due primarily to (a) the organic solvent or oil in the cutback and/or (b) the water in the emulsion or on the aggregate surface which interferes with the formation of an adhesive bond between the aggregate and asphalt. One technique which has been disclosed to increase such adhesivity is set forth in U.S. Pat. No. 2,342,861. The examples of that patent illustrate the addition of a lead soap, specifically lead oleate or naphthenate, to asphalt cutbacks or emulsions to increase their adhesivity for aggregate. Although in all illustrated examples only lead is disclosed as a metal soap to increase adhesivity, the patent suggests that other heavy metal salts of organic acids could be employed including the following metals: Fe, Al, Mn, Zn, Co, Ni, Sn, Ca, Sr, Ba, and Mg. The patent discloses a technique of forming the lead soap by heating a lead oxide in the presence of the desired organic acids. Such lead soaps are then added to the desired asphalt.
Heavy metal salts of high molecular weight organic acids, such as naphthenates or linoleates, have been employed to prevent cracking in blown or oxidized asphalt coatings. For example, U.S. Pat. No. 2,282,703 discloses the use of heavy metals such as cobalt, manganese, iron, lead, vanadium, or zinc dispersed into the blown asphalt for this purpose.
Heavy metal soaps have also been disclosed for use as a dispersant in roofing asphalts to prevent failure of the asphalt due to "alligatoring". U.S. Pat. No. 2,928,753 discloses the polyvalent metal salts of copper, cobalt, or manganese in combination with high molecular weight monocarboxylic acids such as oleic or naphthenic acid. The final disclosed product is an aggregate-free coating of 0.025 inches thick on an aluminum sheet heated so that leveling occurs.
In U.S. Pat. No. 1,328,310, an asphaltic pavement is disclosed in which copper sulfate is added to the asphalt for improved physical properties. Other compounds are disclosed including the sulfates or selenates of aluminum, chromium, manganese, iron, indium, gallium, and the sulfates or selenides of sodium potassium, rubidium, ammonium, silver, gold, platinum or thallium. These compounds are relatively insoluble in the asphalt.
In U.S. Pat. No. 1,505,880, copper slag is added with the aggregate to asphalt to increase the toughness of the resulting pavement composition.
In British Pat. No. 533,977, lead or iron double salts of organic acids are disclosed for the purpose of improving adhesivity of asphalt for mineral aggregate. Also disclosed, incidentally, is that other di- and multivalent metals such as aluminum, chromium, copper and mercury can be used.